1. Field of the Invention
This invention relates to optical fiber coils and to methods of manufacture thereof, and more particularly, to an improved coil pattern.
2. Description of Related Art
Fiber optic sensor coils are used, inter alia, in fiber optic gyroscopes to provide an optical output signal used in determining rotation of a vehicle (e.g. an airplane) about an axis of rotation. A typical fiber optic gyroscope uses three sensor coils to sense rotation about each of three orthogonal axes. The fiber optic gyroscope is typically configured as a Sagnac interferometer including a light source providing an optical signal, a multi-turn coil of optical fiber, referred to as a fiber optic ring, and electronic read-out and control circuits. The optical signal is first applied to an optical beam splitter/combiner which provides two identical optical output signals, each of which is applied to one end of the fiber optic coil. The two optical signals travel through the coil in opposite directions and are recombined at the beam splitter/combiner. A rotation of the fiber optic coil about its wound, or longitudinal axis will result in a phase shift between the counter-propagatory optical signals traveling through the coil. This phase shift is known as the Sagnac effect phase shift. The Sagnac effect can be explained by relativistic theory which shows that a wave traveling through a rotating coil in the direction of rotation requires more time to traverse the path than a wave traveling opposite to the direction of rotation. This time difference is manifested by phase shift interference pattern of the recombined optical signal. In an optical gyro, the magnitude of the phase shift is determined by analysis of the recombined signal as applied to an output optical detector. The detector output is translated into electrical output signals representing rotation.
The phase shift detected at the output detector may be considered as consisting of two parts. The first part is the Sagnac effect phase shift. The other part of the detected phase shift is due to perturbations in the optical fiber caused by environmental factors. The Sagnac phase shift which defines the magnitude and direction of rotation is relatively small, such that any significant phase shift due to environmental factors may obscure an accurate reading of the Sagnac effect phase shift. It is therefore desirable to minimize the effect of environmental perturbations on the detected phase shift of the recombined optical signal.
The two optical signals emanating from the splitter/ combiner in response to the single optical input signal are in phase and are applied at opposite ends of the coiled fiber and traverse the fiber in opposite directions. The undesirable phase shift effects occur when environmental perturbations affect one of the light signals differently than the other. It is generally recognized that environmental perturbations cannot be eliminated, but that their effect can be minimized if these perturbations are applied equally to the counter propagatory light signals. A known approach to reducing the effect of environmental perturbations is to build a symmetry in the sensing coils. Known sensing coils include dipole, quadrupole, and octupole windings. In these coils the midpoint of a length of optical fiber is placed near one side flange of a spool and the two optical fiber segments emanating from the midpoint, referred to as the forward segment and the reverse segment, are then wound around the coil in opposite directions. In the case of a dipole, the forward and reverse segments are wound on the spool in alternating layers. In the quadrupole, a layer of the forward segment is followed by two layers of the reverse segment, followed by two layers of the forward segment, and so on. In an octupole configuration, two layers of the reverse segment are sandwiched between two layers of the forward segment followed by another set of four layers in which two layers of the forward segment are sandwiched between layers of the reverse segment. All of these various configurations are attempting to introduce a symmetry such that an environmental perturbation of the coil will affect the counter-propagating light signals in the same manner. However, when the coil is built up of alternating layers or alternating pairs of layers of the forward and reverse segments, the forward and reverse segments are not necessarily affected in the same way by the environmental perturbations. This may be better understood by considering the nature of the environmental perturbations.
Environmental perturbations may be due to mechanical strain, vibration, shock and temperature changes. It is known, that predominantly two types of temperature perturbations have to be dealt with, namely, those due to radial temperature gradients and those due to axial temperature gradients. A third gradient type, transverse to the wound (longitudinal) axis of the spool, is a less significant problem. As the name implies, with a radial temperature gradient, the temperature varies radially such that an optical fiber segment comprising a portion of the innermost layer of the coil is at a different temperature than fiber segments in layers which are a distance removed from the core of the coil. An axial gradient extends along the wound axis of the spool. It is therefore desirable to avoid significant axial and radial distances between segments of the optical fiber which are the same distance from the center point of the length of the fiber such that corresponding segments of the forward and reverse segment of the coil experience the same environmental perturbations.